Biconical horn and reflector antenna



' `INVENTOR. John Hagen At'rorney J. P. HAGENy BICONICAL HORN ANDREFLECTOR ANTENNA Filed May 31, 1968 Aug. l4, 1970 United States PatentO 3,523,298 BICONICAL HGRN AND REFLECTOR ANTENNA John P. Hagen, StateCollege, Pa., assignor, by mesne assignments, to the United States ofAmerica as represented bythe Secretary of the Navy Filed May 31, 1968,Ser. No. 733,459 Int. Cl. H01q 19/14 U.S. Cl. 343-775 6 Claims ABSTRACTOF THE DISCLOSURE The invention described herein may be manufactured andused by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon orvtherefor.

This invention relates to antennas and more particularly to lightweightlarge aperture directional antennas for use in astronomy and satellitetracking systems.

One of the major problems encountered in the design and fabrication oflarge aperture antennas has been that of maintaining the precise shapeof all radiating and reflecting surfaces as the antenna is subjected todynamic and static structural loading. In the most common of the largeaperture antennas, the parabolic antenna, these surfaces must beaccurate within a small fraction of the operational wavelength of thearray. The feed structure of the paraboloid, which is usually supportedfrom a central portion of the parabolic reflector, must also beprecisely positioned with respect to the focus of the parabolicVreector. Any displacementl of this feed from the focus will not onlyresult in additional deformation of the reflecting surface but alsodecrease the antenna gain and pointing accuracy.

The antenna of the present invention is a lightweight, relatively lowcost, stable structure. Its geometry is that of an extended biconicalhorn with a cooperating peripheral reflector which redirects theradiation emanating from the mouth of the horn in a single direction.The weightto-size ratio of the present antenna is substantially reducedbecause its major structural components are relatively thin metalmembers that are placed'under tension. This tension stabilizes theirshape by preventing llexure. These metal members in one embodiment forma geometric structure resembling a rhombus revolved around its minordiagonal. To this structure we give the name double cone. Theconfronting surfaces of two of these double cones, when suitably spaced,form the biconical horn portion of the present invention.

The annular reflector which cooperates with this horn is also oflightweight design. As in the structural design of the double cone,stability of the shape of the reflector is achieved by tensioning thestructure.

The biconical antenna is fed at its center of gravity thus eliminatingnot only the problems of exure found in construction of parabolicreflectors but also beam pointing discrepancies due to displacement ofthe feed from the focus.

3,523,298 Patented Aug. 4, 1970 It is accordingly a primary object ofthe present invention to provide a lightweight antenna of relativelylarge size whose major structural elements are fabricated from thinmetal members.

Another object of the present invention is to provide a lightweightantenna wherein the shape of the critical radiating or reflectingelements are stabilized by having these elements maintained undertension.

A further object of the present invention is to provide a biconical hornantenna with an annular conical reliector to achieve a sharpunidirectional antenna pattern.

A still further object of the present invention is to provide means fordirecting the flow of energy through a biconical antenna in such a waythat energy arriving at the coupling means of the antenna from all partsof the annular aperture will be of the same polarization.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description thereof whenconsidered in conjunction with the drawing.

The single figure shows an antenna assembly which consists of threesubassernblies: double cones 1 and 2, whose confronting surfaces definea biconical horn; an annular conical reflector 3, which imparts aunidirectional beam pattern to the apparatus; and a combined internalreflector 12 and feed 15 combination. The term, double cone, hereinafterused, is intended to describe the geometric solid formed by revolving arhombus about its minor diagonal. When two such double cones are alignedalong a common axis and spaced apart, their confronting conical surfacesdefine a biconical antenna.

The upper double cone 1 of the antenna assembly is constructed fromrelatively thin metal members 6 and 9, each of which have a similarconical shape. Both cones may be prefabricated or they may be formed byjoining two like circular planar sheet members together at theircircumferential edges and thereafter displacing their centers anappropriate distance apart. In the latter case, a tapered rigid ring 8of appropriate diameter is secured between the rim portions of two suchsheets. The taper of this ring member determines the size of the doublecone assembly.

In order to separate the centers of these sheets by controllableamounts, an extensible hub structure 13 is secured between these points.This axial movement, it will be appreciated, has the effect of placingunder tension those portions of sheets 6 and 9 which are between therigid ring member and the stationary central hub. It is this tension, asnoted hereiubefore, which stabilizes the subassernbly, while at the sametime permitting it to be fabricated of otherwise normally flexiblemembers.

The hub structure of cone 1 includes an extensible strut 13 adapted tocoact with pads 11 and 12 which distribute the mechanical load of sheets6 and 9 attached thereto. Adjustment of the tension in sheets 6 and 9 isaccomplished by increasing the separation between pads 11 and 12. Pad12, in addition to providing anchoring means for sheet 6, includes alower surface which functions as an internal reflector. Pad 12 may bethreaded to receive strut 13 which when turned increases the separationbetween pads 11 and 12. Turning of strut 13 is accomplished by turningthat portion of strut 13 which passes through and projects above pad 11.It will be appreciated that as the size of the antenna is increasedother tensioning devices may be employed to form the uniform doubleconical surface.

The degree of regularity of the reflecting surfaces of the double conewill depend on the guage of the material, its elastic constant, itsdensity, its extent and the force applied thereto.

Double cone 2 is similar in construction to ,cone 1. Sheets 7 and 10 aresecured to ring 18 at their peripheral sides and edges and centrally toa hub consisting of hollow strut 14 slidably mounted in hollow pad 16.Here the hub is hollow to allow access of coupling means to the throatof the biconical horn. A horn 15 is mounted through the hollow hub andis secured to the inner face of hollow strut 14 to provide for couplingof electromagnetic energy into and out of the antenna.

While the ligure shows the use of mechanically actuated, variable lengthstruts as the apparatus for developing tension in the sheets,conventional pneumatic counterparts may be employed in place of thestruts. In its collapsed form, the double cone under tension willresemble two drumheads. It is possible to feed the drumheads at theircenters and derive an omnidirectional radiation pattern.

Double cones 1 and 2 are separated and held in alignment along a commonaxis by a multiplicity of insulated posts distributed around theperiphery of the horn. Alternatively, this separation may be achieved bythe utilization of a cylinder constructed of a low-loss dielectricmaterial in place of the posts. A further means for maintaining theproper separation includes the use of a toroidal lens made of a low-lossdielectric material in place of the posts.

The second subassembly, the reflector, redirects the omnidirectionaloutput of the biconical horn into a single direction. When cones 6 and 7are excited at their confronting apices by cone feed 15, maximumradiation is obtained at right angles to axis 5, the common axis of thedouble cones. The pattern of energy emanating from the mouth of thisbiconical radiator is omnidirectional in a plane 4 perpendicular to axis5 and passing through the center of the array.

In the transmitting mode, an annular conical reflector 3 is positionedadjacent the mouth of the biconical radiator formed by cones 6 and 7 toreect the omnidirectional radiation emanating therefrom into aunidirectional beam 30 parallel to axis 5. This beam issues forth froman aperture which is the annulus, r, formed by the periphery of cone 6and that part of reliector 3 adjacent this peripheral edge. Since thepolarization of the wave front appearing in the annular aperture r mustbe maintained the same al1 over the aperture, the Wave propagating inthe biconical radiator must be cylindrical. This cylindrical wave isreflected by reflector 3 and emerges from the antenna as a plane wave ofone polarization. Reector 3 is itself the truncated portion of a rightisosceles cone, the surface of which forms an angle of 45 with respectto plane 4. The surface of this reilector may include a metallic sheetplaced under tension by supports 22 and 23 secured to base 21. Thetensioning of the metallic sheet also adds stability to the array. If,however, the wave were guided from the transmitter (not shown) throughthe device to the aperture r, the polarization on one side of theantenna would oppose that on the other side and a null would appear inthe direction of propagation rather than the desired maximum. Thisdiiculty is obviated by the third subassembly, the coupling retiectorand horn, 12 and 15.

Horn 15 establishes a plane wave front at its opening or mouth 25. Thisplane wave propagates out of horn 15 and is reflected by the lowerportion-of pad 12 which extends to, but not into, mouth 25. This lowerportion is metal coated and is in lthe form of a right isosceles cone.If the apex of cone 12 extends into mouth 25, a plane Wave is preventedfrom forming. If this apex is spaced from the mouth, maximum rellectionof the plane wave will not be achieved. After reflection by the rightisosceles cone, the Iplane wave propagates down the biconical radiatoras the required cylindrical wave.

In the receiving mode, reflector 3 directs an incoming plane wave 30towards the center of the array where it is reected by the rightisosceles cone. The polarization of the incoming Wave is preserved as itis reconstructed at opening 25 of horn 15 by conical pad 12.

The beamwidth of the antenna will be determined by the outside diameterof the array. This beamwidth is given by -where 0=beamwidth in degreesD=the aperture ofthe antenna Azoperational wavelength of the antenna.

The amount of energy in the minor lobes of the antenna pattern will bealfected by the width of the annular aperture r. The gain of the antennawill be proportional to the area of this aperture. For large apertureantennas this gain G is given by where G=the gain over an isotropicradiator A=area of the aperture =operational wavelength of the antenna.

The antenna thus constructed may be mounted and gimballed in theconventional manner. Trunnions 24, mounted on base member 21, areprovided to receive a yoke 26 for this purpose. Because of thelightweight stable construction, tortional loading produced by steeringor the encountering of wind will be minimized and Iiexure of theradiating surfaces reduced.

It will -be appreciated that an alternate method of constructing themetallic double cones lies in the substitution of metallic spokes (notshown) secured to the central hub and the peripheral ring for the thinmetal members. An array thus constructed, in addition to having afurther reduced weight-to-size ratio, also reduces the elfect of wind onthe array.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. An antenna comprising:

a pair of hollow, double cones in spaced relationship such that theirconfronting surfaces form a biconical horn assembly,

the conical surfaces of each of said pair of double cones beingconstructed from relatively thin, lightweight material;

means for placing each of said double cones under tension such that theconical surfaces of said cones are made rigid and resist deformation,the throat portion of said horn assembly corresponding to theconfronting apices of said spaced double cones; and

means for coupling electromagnetic energy into and from said throatportion.

2. An antenna comprising:

a pair of double cones in spaced relationship such that theirconfronting surfaces form a biconical horn assembly, the throat portionof said horn assembly corresponding to the locus of the confrontingapices of said spaced double cones; and

means for coupling electromagnetic energy into and from said throatportion, said coupling means including a right isosceles cone secured toand centered on one of said confronting surfaces and means for forming aplane wave at the center of the other of said confronting surfaces, suchthat plane waves formed at the center of said other surface arereflected by said right isosceles cone outwardly through said biconicalhorn.

3. The antenna as recited in claim 2 wherein said plane wave formingmeans is a conical waveguide positioned so that its mouth projectsthrough said other confronting surface into said throat such that thecenter of said mouth lies at the apex of said right isosceles cone.

4. The antenna as recited in claim 2 further comprising means located atthe open end of said biconical horn for reflecting radiation propagatingoutwardly therefrom parallel to the axis of alignment of said doublecones and for reecting radiation incident on said reflecting surface tosaid coupling means.

5. The antenna as recited in claim 4 wherein said reflecting means is atruncated right isosceles conical surface under tension positioned suchthat its edge of smaller circumference coincides with the peripheraledge of one of said double cones and such that the portion of theconical surface adjacent the peripheral edge of the other of said doublecones is spaced therefrom.

6. In a biconical antenna the combination of:

a pair of axially aligned metallic double cones,

each of said double cones comprising two circular plates of equaldiameter joined at their peripheral edges, a rigid ring attached to saidjoined edges and means adjustable in length and coacting with thecenters of said plates to separate them so as to form a double conical 5surface under tension;

means for spacing said double cones such that their adjacent surfacesform a biconical horn; and means at the center of said horn to coupleelectromag- 10 netic energy into and out of said antenna.

References Cited UNITED STATES PATENTS 2,471,021 5/ 1949 Bradley 343-77415 2,532,551 12/1950 Jarvis 343-774 ELI LIEBERMAN, P rimaryExaminer U.S.Cl. X.R.

